1. Field of the Invention
This invention relates generally to a receiver digital processing method and associated Global Positioning System (GPS) receiver, and particularly, to a method for processing weak indoor signals in the presence of cross-correlation or continuous wave interference and associated GPS receiver.
2. Description of the Prior Art
A conventional existing GPS receiver contains an antenna and an analogous front-end (AFE) followed by a digital section having dedicated signal processing circuitry and a digital CPU with related program and data memory and external data interface controllers. The antenna together with the analogous front-end intercept, select (band-pass filter), amplify GPS signals, convert them to a convenient intermediate frequency (IF) normally ranging from DC to several tens of MHz. To perform frequency conversion, the AFE utilizes a reference frequency from a stable reference oscillator. The AFE typically outputs digitized samples of a combination of signals and accompanying noise at IF. The frequency of sampling the AFE output is selected according to the Nyquist criterion, and for the Clear/Acquisition (C/A) GPS signal component is, at least about 2 MHz. A number of bits in digital AFE samples varies from one to three or four bits.
A digital section of the GPS receiver contains several correlate channels that perform correlation processing of several GPS satellite signals in parallel. GPS signals employ phase shift keying modulation with pseudo-random noise codes, see, for example, “Understanding GPS: Principles and Applications. Edited by Elliott D. Kaplan. Artech House, Boston, London, 1996, pp. 83-97”. Received signals are characterized by a priori uncertainty of signal parameters: its code phase due to unknown (or not ideally known) time of the signal coming to the receiver, and its carrier frequency due to unknown (or not ideally known) Doppler shift and the reference oscillator frequency drift. Signal search in a GPS receiver, i.e. resolution of the above-mentioned uncertainty, requires time. Many applications of GPS need receivers that are capable of acquiring signals rapidly in difficult signal environments. For example, this can be reception of weak GPS signals indoors and in urban canyons. A short time to acquire these weak signals is important both from a direct viewpoint of a user requirement to get the first position fix as soon as possible, and from the viewpoint of supply energy reduction as a result of a short time-to-first-fix (TTFF).
When receiving weak GPS signals, for example, in urban canyons, indoor or under trees, a common problem appears associated with the fact that the signals can arrive to the receiver having significantly different strength. The problem is known as cross-correlation interference from stronger signals to affect the processing of weak signals. GPS signaling (its civil C/A component) was designed to be safely processed only if signals from other satellites are not stronger than by about 23 dB, or even less, to have a margin. General measures to mitigate the effect of cross-correlation interference are known. For example, the U.S. Pat. No. 6,236,354 to Krasner describes three techniques to decrease the effect of cross-correlation.
The 1st technique makes use of the evaluated parameters of a strong signal acquired by the receiver, reproduces its waveform, appropriately scales it, and subtracts it from the signal combination at the input before any signal processing to remove the interference component from the input signal. Potentially, this 1st technique is the most effective among the described ones. But implementing this technique as it is described in the U.S. Pat. No. 6,236,354 to Krasner has several disadvantages. First, the compensation of a strong signal can not be full, as there are two contradicting tasks: to suppress the strong signal that interferes with the reception of weak signals, and, simultaneously, to proceed tracking for the strong signal to use it in a navigation solution and continue fine tuning to suppress it. Second, in trying to deeply suppress the strong signal, it is easy to overcompensate it so that the replica becomes stronger than the original signal. There is a serious risk that continued tracking follows the subtracted replica, not the signal. The technique is not robust enough and needs improvement.
The 2nd and the 3rd techniques of mitigating cross-correlation according to the U.S. Pat. No. 6,236,354 to Krasner make use of the evaluated parameters of a strong signal acquired by the receiver, predict the cross-correlating effect from the strong signal to the anticipated weak signal, and correct the correlations accumulated for this weak signal. The difference between the techniques is that the 2nd one comprises subtracting the predicted effect from the accumulations, and the 3rd one simply discards potentially injured accumulations. A disadvantage of the 2nd and the 3rd techniques is their high computational requirements to predict the cross-correlation for all possible combinations of signals' PRN codes, code phase differences, and Doppler frequency differences. Possible simplifications reduce the effectiveness of the techniques. Another disadvantage of the 3rd technique is that discarded accumulations may contain the desired signal correlations, and the probability of this occasion rises with the strength of the interfering signal or, equally, with a decrease of the weak signal power. The above-mentioned disadvantages of the 1st technique proscribe effectively combining the techniques, for example, the 1st and the 3rd ones, and thus do not allow relaxing requirements of the 3rd technique.
A common disadvantage of all listed-above cross-correlation interference suppression techniques is the complexity of involved hardware used to compensate for the interfering waveforms at the receiver input, and of the calculations required to accurately predict the parameters of interfering signals. New, simple and robust methods of cross-correlation interference suppression are demanded to meet the continuous growth in requirements of GPS receivers.